1. Field of the Invention
This invention relates to a method for fabricating semiconductor devices, and more specifically to a method for forming patterns for semiconductor elements on a silicon semiconductor substrate having a protective coating.
2. Description of the Prior Art
It is generally well known to those skilled in the art that, in the processes of fabricating a transistor or IC, patterns are formed on the semiconductor substrate or on the protective coating of the semiconductor element.
The most generally used method for forming patterns is as follows: a substrate of silicon, germanium, or other similar semiconductor material is first oxidized to form the silicon dioxide coating on the surface of the substrate by heating it in an atmosphere containing oxygen. A layer of a photosensitive material, such as, for example, a photosensitive polymer (photoresist) is next applied to the dioxide surface. An opaque mask is then applied to the element to block off the edge areas and the unmasked areas are exposed to actinic radiation such as ultraviolet light and hardened. The unexposed areas of the photoresist are removed with an organic solvent, and thus the silicon dioxide surface is exposed. The element is now heated to harden the irradiated areas and then the entire substrate is treated with an etching solution, such as hydrofluoric acid to dissolve the exposed silicon dioxide surface uncovered with the photoresist. Afterwards the hardened photoresist above is removed with an organic solvent. Subsequently the diffusion processes of another metal material is applied to the uncovered surface portions of the element.
The photolithographic operation described above is generally known to those skilled in the art as the photoetching technique. In fabricating a semiconductor device, several diffusion process operations are applied, depending upon the type of the semiconductor device. It is, therefore, required to repeat the cycles of applying photosensitive material, exposing, developing, etching the dioxide coating, removing the unexposed portion of the photosensitive material, and diffusion process each time, and accordingly much time and high grade technique is required to practice these operations. In addition, these procedures have a disadvantage in that the hardened portions may be damaged by the operation of removing the unhardened portions.
To eliminate these disadvantages, there have been disclosed several improved photoetching techniques, for example, as described in U.S. Pat. No. 3,095,332; No. 3,095,341; No. 3,122,463; No. 3,255,005; No. 3,346,384; No. 3,471,291; No. 3,489,564; No. 3,494,768 and French Pat. No. 1,394,467.
Directing our attention to the method of forming patterns on the silicon dioxide coating of a silicon semiconductor material, which is disclosed in the aforementioned patent specifications, a silicon slice having silicon dioxide coating is disposed in a particular ambient of a composition which is decomposed by light into the material which etches the silicon dioxide, and exposed to ultraviolet radiation through the photomask having a required pattern for etching the silicon dioxide coating to form the pattern with a suitable depth. That is, the exposure and etching are practiced simultaneously.
To explain this more specifically, a silicon slice having a silicon dioxide coating is disposed in a gaseous ambient of a fluorine compound, such as F.sub.2 O (difluoroxide), the fluorine of which may be set free by exposure, and exposed to actinic radiation through photomask, or while being exposed in a gaseous ambient of a fluorine compound, such as fluorobenzene, the silicon slice is cooled to form a liquid state film of fluorobenzene on the surface of the silicon dioxide coating, and, subsequently, exposed to actinic radiation through the photomask.
In this method, however, as the fluorine compound is supplied continuously between the photomask and the silicon dioxide coating, it is very difficult to initiate a reaction between the product of the photodecomposition or fluorine, which is in a fluid state, in the entire surface of the pattern uniformly in a short time. Moreover, it is also difficult to obtain an etched configuration having a sharp line definition.
In the case of forming a liquid state film and irradiating light thereof, considering the surface tension between the liquid and the silicon dioxide coating, and maintaining the liquid state film on the surface of the silicon dioxide coating, problems prevail. For example, there is the problem of how to accomplish the cycle of liquefying the unexposed gas and removing the liquid state film, including the fluorine reacted with the silicon oxide in the entire surface to be etched uniformly, and how to make an apparatus for achieving the same. It is, therefore, readily understood that, according to this method, it is difficult to form accurate patterns due to the fact that the light for exposure is scattered by the gas and liquid flowing between the photomask and the silicon dioxide coating, and it is especially difficult to obtain exceedingly accurate microscopic patterns, which are required for fabricating IC or LSI.